This study presents the morphological description and histochemical characterization of gill filaments of the Brazilian endemic bivalve Diplodon expansus, aiming to broaden the morphological knowledge of this species and establish the structure of the gills that will serve as control in histopathological studies applied to biomonitoring. The gill filaments are divided into three zones: frontal, intermediate, and abfrontal. In the center of the filament, haemocytes circulate through the haemolymph vessel, which is internally lined by endothelium. The frontal surface of the filament is covered with cilia, the lateral surface exhibits aquifer ducts, and the abfrontal surface presents ciliated and nonciliated cells. The epithelium of the filaments is composed of ciliated cells, nonciliated absorptive cells, and mucocytes. The support of the filaments is made by two specialized structures called skeletal rod and skeletal loop. Based on the obtained information, the gill filaments of the studied species present some peculiar characteristics that are not yet reported in detail in the literature such as the simultaneous presence of skeletal rod and skeletal loop. On the other hand, the general constitution of the filament is similar to that described for both marine and limnic bivalves and seems to be suitable for ecotoxicological studies.